The process of forming a pyrolytic carbon coating utilizes high-temperature gas-phase dissociation of low-molecular-weight saturated or unsaturated hydrocarbons which undergo polymerization and/or dehydrogenation reactions to ultimately form a finely particulate form of carbon. When this occurs in a fluidized bed of particles in which the hydrocarbon itself, or combined with an inert diluent gas, serves as fluidizing medium, the particles will receive a pyrolytic carbon coating, the nature and quality of which depend on several process parameters including such factors as the nature of the pyrolysis gas, deposition temperature, deposition rate, and uniformity of fluidization. For specific details, reference is made to an article by R. J. Bard et al., "Pyrolytic Carbons Deposited in Fluidized Beds at 1100.degree.-1400.degree. C. from Various Hydrocarbons," Carbon, 1968, pp. 603-616. See also, U.S. Pat. Nos. 3,399,969, 3,977,896, 4,546,012, and 4,594,270, all of which are incorporated herein by reference.
There have been many devices proposed for supplying gas in fluidized bed coaters for the purpose of uniformly distributing the fluidizing gas throughout the mass of such beds. Typically, such coating operations have recently been conducted in a relatively small apparatus utilizing a graphite or graphite-lined tube several inches in internal diameter with a lower cone-shaped section which serves as an inlet passage for a fluidizing gas stream that typically contains a decomposable hydrocarbon and an inert gas, such as argon, nitrogen, or helium. While this can be an effective way of uniformly coating with small charges of particles, effectiveness can decrease sharply as the amount of charge increases, causing the employment of larger diameter chambers for larger charges. When this is done, non-uniform fluidization and non-uniform coating can become a severe problem.
The flat-bottom, or flat-bed coater is a device that has the potential of producing a more uniform coating on a medical-device component within a bed of particles, compared to conical-bottom coaters, when viewed on a microscopic scale. It can yield finer silicon (Si) distribution and less banding than does a conical-bottom coater. Flat-bed coaters commonly utilize either porous frits or distributor plates containing a plurality of drilled holes to define the bottom or lower wall of the coating chamber through which both the fluidizing and coating gases enter.
However, one of the major drawbacks of current flat-bed coaters is the tendency of the coating gas component to clog the gas-distributor pores or holes of the distributor plate, thereby modifying the flow of gas upward into the bed and causing less uniform fluidization prior to the completion of the relatively long coating run, which results in coatings of less uniformity. While the distributor plate is not heated directly during the coating process, radiation from the zone of coating and fluidization often heats areas of the plate to a temperature sufficient to pyrolyze the hydrocarbon. More specifically, the temperature at the surface boundary of the gas-distributor plate holes, especially the peripherally outer holes of the plate, becomes sufficiently high enough to decompose the coating components of the coating-fluidizing gas. This often results in clogging of the outer holes of the plate, either by coating-over the top of the hole or by plugging the inside of the hole, which in turn causes the fluidized bed in that area to have less levitation. As a result, the entire bed action can change in a manner which can have an undesirable effect on the material properties of the coating.
Flat-bed coaters used for coating objects useful as components in prostheses or other medical devices require relatively long coating periods and cannot tolerate the distributor plate holes clogging, which affects the uniformity of fluidization. Thus, since the degree of coating uniformity is a direct function of the degree of uniformity of fluidization, it is an object of this invention to provide apparatus for maintaining a high degree of uniform fluidization of levitated particles for a long enough time period to coat medical-device components. This is accomplished by utilizing a coating-fluidizing gas supply system that prevents the coating-over or plugging of gas-distributor holes in a flat plate at the bottom of a flat-bottom fluidized bed coater.